This invention relates to a circuit arrangement for generating a clock frequency in a smart card interface.
Smart cards are often used in mobile telephones, amongst other devices. For example, mobile telephones using the German Netz-C system employ smart cards. In this system, the identification data of the mobile telephone are recorded on the smart card, such data being e.g. a call number and a code number as well as possible other data, which is then read into the mobile telephone via the smart card interface when needed. The pan European GSM (Groupe Speciale Mobile) mobile telephone system which is being adopted at present has also taken account of the use of a smart card.
The smart card interface is defined in the International Standards Organization (ISO) recommendation 7816-3. In the GSM system, the smart card implementation is based on this ISO recommendation, however some additional conditions have been set for a smart card implementation by the GSM recommendation 11.11.
The ISO recommendation 7816-3 defines various communication modes for a smart card interface. There is a basic distinction between the "Answer to reset" (ATR) mode and normal communication modes. The ISO recommendation also mentions that various communication protocols, i.e. signal systems used in communications, can be applied to the communication modes. One communication mode is defined in the ISO recommendation; it is called "T=O". This communication mode includes a parity check and a retransmission in the case of an error.
Universal Asynchronous Receiver Transmitter (UART) circuits are preferably used in the usual communication between the various equipment units. The protocol defined for the communication mode "T=O" however involves the special problem of the parity having to be checked in the input bit current and the possible error having to be detected 0.5 bits after the reception of the parity bit. Because of this requirement, a usual UART circuit cannot be used for the communication transmission of a smart card interface. This protocol involves another problem with regard to the definition in the recommendations, which requires a ratio of 372 between the clock frequency of the smart card and the data rate transmitted by the interface. The clock frequency for the data reading in UART circuits is usually 1, 16 or 64 times the data rate. When using the coefficient 1 in the implementation, the UART circuit would operate if the first requirement as to the error detection rate were satisfied. When using the coefficient 16 in the implementation, no problem would occur if the coefficient 16 were a factor in the number 372. However, this is not the case. The problem can be solved by adding to the system the frequency by means of which this problem is overcome. However, this solution involves the problems of committing oneself to a given clock frequency of the smart card, which can be altered during the use of the device, e.g. a mobile telephone, only by changing the externally fed clock frequency of the UART circuit accordingly. Nevertheless, such a situation would entail servicing the device, opening of the device and hence considerable costs.